1. Field of the Invention
This invention relates to a proportional combustion control device for adjusting a supply of fuel to a combustor for a boiler, a hot water unit, etc. to control combustion.
2. Description of the Prior Art
FIG. 1 is a schematic view showing a conventional combustion control device. In FIG. 1, reference numeral 1 designates a fuel tank, and 2 designates a fuel supply pipe having one end connected to the fuel tank 1, an extreme end of which is provided a nozzle 3 composed of a fixed orifice as shown in FIG. 2. Reference 4 designates a pump for feeding under pressure fuel 5 to the nozzle 3 provided in the midst of the fuel supply pipe 2, and 6 designates an opening and closing valve for controlling a supply of fuel to the pump 4.
The operation of the device will be described hereinbelow. When the valve 6 is opened to actuate the pump 4, fuel 5 in the fuel tank 2 is fed under pressure to the extreme end of the fuel supply pipe 2, and the fuel 5 is jetted in an atomized form out of the nozzle 3.
The conventional proportional combustion control device is configured as described above. In the case where a boiler less than 40000 Kcal and an instantaneous hot water maker for home are used and fuel is oil, fuel supply pressure has to be controlled for proportional combustion control since the nozzle 3 is a fixed orifice. However, when pressure is excessively lowered, particles when oil is jetted become large, flame becomes large, firing becomes hard or soot is produced. As the result, the fuel supply pressure cannot be excessively lowered. The number of opening and closing of the valve 6 per unit time is changed to control the fuel supply. However, the combustion is turned into indirect combustion, posing a problem in that firing noises, "bang", occur at the time of firing.
A conventional combustion control device in which fuel is limited to oil alone is shown in FIG. 3. FIG. 3 shows a boiler control device shown, for example, in Japanese Patent Laid-Open No. 55 (1980)--3600 publication. In FIG. 3, a boiler control device 20 is shown. The boiler control device 20 is provided with an oil burner 22 having a combustion chamber 24. Fuel oil 26 is supplied from a supersonic nozzle 28 to the combustion chamber 24, and a nozzle 28 jets the fuel oil 26 through a hole 30 of a flame holder 32. The oil burner 22 is provided with a heat exchanger 34 as shown.
The supersonic nozzle 28 is a vibration device having a closure element of a ball valve, said element being moved by vibrations of the vibration device from a position at which an oil path is closed to a position at which the oil path is opened.
When the supersonic nozzle 28 electronically controls a ratio of off time to on time, the boiler control device 20 acts so that it is switched to on and off with high frequency.
Reference numeral 44 designates a thermostat for detecting a temperature of liquid to be heated. Information of the thermostate 44 is transmitted to a control device 42 through a conductor 46. The control device 42 controls a speed of a ian 52 and controls a quantity of fuel oil to the supersonic nozzle 28 through a fuel oil supply pipe 60.
More specifically, this boiler control device comprises an oil burner, an electrically operated oil jetting device provided with vibration means having a ball valve closure element which is moved by vibration from a position at which an oil path is closed to a position at which the oil path is opened in order to supply a suitable quantity of oil to a combustion chamber of said oil burner, and a switch device for controlling on and off time of said oil jetting device, whereby combustion is controlled in a simple method without using a complicated expensive burner nozzle. However, in the conventional device, fuel is limited to oil alone as mentioned above, and in case of gas, there is a drawback that it cannot be applied.
Next, in the case where gas is used as fuel, there is no inconvenience encountered in case of oil as described above and no measure thereto is necessary. However, a turndown ratio of 1/5 to 1/10 is necessary in order to improve the fuel supply control. If this is effected by a governor, fuel supply pressure should be set to 1/25 and 1/100 at the turndown ratio of 1/5 and 1/10, respectively. Accordingly, even if the fuel supply pressure is 100 mmH.sub.2 O, the pressure is 1/100, and actually, it should be changed to 1 mmH.sub.2 O. Therefore, it is impossible to control 1:10 by a single pressure-feed means. In view of this, it is contemplated that two pressure-feed means are used, and the range of the pressure-feed means is divided for use.